Portable electronic devices are becoming smaller and smaller, which accordingly causes the volume and capacity of their batteries to decrease. In order to improve endurance of the batteries, power supply modules of these electronic devices are required to provide a stable output voltage when the battery voltage varies in a wide range. Four-switch buck-boost converters thus are widely used in these applications.
A four-switch buck-boost converter typically requires the input current, the output current and/or the inductor current to be sensed in some way to control the power switches and/or to protect the converter. Conventionally, the inductor current is sensed by placing a sensing resistor in series with the inductor, the voltage across the sensing resistor is detected using a sensing amplifier to produce a signal proportional to the inductor current. This type of current sensing approach has the drawback of adding power dissipated in the sensing resistor thereby reducing the overall efficiency of the buck-boost converter.
Alternatively, it is known to use one of the power switches as a sensing resistor and detect the voltage drop across the internal resistance between drain and source of the MOS device (RDSON). This alternative approach overcomes the efficiency reduction caused by a sensing resistor. Nevertheless, since the four power switches in the buck-boost converter are controlled and driven for three different working modes including a buck mode, a boost mode and a buck-boost mode, the current flowing though one of the power switches is simply a part of the inductor current, so multiple current sensing circuits are required to respectively detect the current flowing though each power switch. However, each of multiple current sense circuits is dependent on the different circuit configurations, which increases the cost and size of the system application, and also increases the complexity of a control circuit and the current sense circuits.
Accordingly, a simpler current sensing circuit used for a four-switch buck-boost converter is required to at least address one or some of the above deficiencies.